Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys

Definitions

• This invention relates to a production method of a steel pipe excellent in corrosion resistance and weldability. More particularly, this invention relates to a method of producing easily and at a low cost a steel pipe which has a high corrosion resistance in an environment containing wet carbon dioxide and a small amount of wet hydrogen sulfide, has also excellent weldability and can be used as oil well pipes for the exploitation and production of petroleum/natural gases and line pipes for the transportation, for example.
• Petroleum and natural gases produced in recent years have become more and more of the type which contains wet carbon dioxide and hydrogen sulfide. It is well known that under such an environment, carbon steels and low alloy steels corrode remarkably. To transport such corrosive petroleum and natural gases, it has been customary to add a corrosion inhibitor as an anticorrosion countermeasure. In the case of offshore oil wells, however, it is enormously expensive to add and recover the corrosion inhibitor, and the use of the corrosion inhibitor has become more and more difficult due to the problem of ocean pollution. For these reasons, recently, a need for corrosion-resistant materials which do not need the addition of a corrosion inhibitor has become greater.
• martensite steel oil well pipes typified by the AISI420 steel have been generally produced in the past as seamless steel pipes by a seamless steel pipe rolling method.
• the seamless steel pipes involve the problems that a production yield and productivity are extremely low and the production cost is extremely high.
• the steel pipe In the case of the martensitic stainless steel pipes produced by the seamless steel pipe production method, the steel pipe must be subjected to quenching and tempering heat-treatments after pipe making, and this is one of the causes for the high production cost of the seamless steel pipes.
• low carbon martensitic stainless steels which reduce as much as possible the C or C and N contents so as to improve the corrosion resistance or weldability, the steel pipes cannot be produced easily by the seamless steel pipe rolling method.
• Japanese Unexamined Patent Publications (Kokai) No. 4-191319 and No. 4-191320 disclose a method of producing a steel pipe from a low carbon martensitic stainless steel
• Japanese Unexamined Patent Publications (Kokai) No. 4-99127 and No. 4-99128 disclose a method of producing a low carbon martensitic stainless steel pipe.
• Japanese Unexamined Patent Publication (Kokai) No. 5-263139 describes a method of producing an oil well pipe containing 12 to 14 wt% of Cr as an electric resistance seam welded steel pipe.
• these methods require heat-treatment such as normalizing and tempering after the steel pipe is made, and involve the problems that the production cost is high, and oxide scales are formed on the steel pipe surface.
• the present invention aims at providing a method of easily producing, at a low cost, a steel pipe having excellent corrosion resistance in a carbon dioxide-containing environment, etc., and also having excellent weldability.
• the gist of the present invention resides in the following points (1) to (7).
• the present invention solves the various problems with the martensitic stainless steels typified by the stainless steel AISI420 steel that have been examined in the past as the corrosion-resistant materials for petroleum and natural gases containing large quantities of carbon dioxide, and is directed to make it possible particularly to secure a high strength necessary for line pipes and oil well pipes, to restrict the rise of the hardness of the welding heat affected zone and to improve the corrosion resistance and weldability.
• the present invention limits the range of the chemical compositions of the steel from the aspect of the corrosion resistance and weldability, and optimizes a hot working condition of a raw steel sheet rolling process and a pipe making process and a cooling condition after hot working.
• Si as a deoxidizing agent and a strengthening element to a steel containing 7.5 to 14.0% of Cr is effective.
• the Si content is limited to the range of 0.01% to less than 1.2%.
• the necessary strength can be obtained by the combination of other alloy elements and the production condition, a large quantity of Si need not be added, and the Si addition quantity is reduced preferably to not more than 0.2% as the necessary and sufficient amount for deoxidation.
• Mn is necessary as the deoxidizing agent for a steel containing 7.5 to 14.0% of Cr, and at least 0.02% of Mn must be added. Mn is also a useful element for converting the metallic structure to the structure mainly consisting of martensite. If the Mn content exceeds 3.0%, however, the effect of addition gets into saturation, and the excessive Mn content induces difficulties in steel making. Therefore, the upper limit of the Mn content is limited to 3.0%.
• the Cr content is limited to 7.5 to 14.0%.
• At least 0.005% of Al must be added as the deoxidizing agent.
• the upper limit of the Al content is set to 0.5%.
• the C content is limited to not greater than 0.03%.
• N lowers the toughness of the weld portion and remarkably raises the hardness of the welding heat affected zone. Therefore, the N content is limited to not more than 0.02%.
• the C content must be limited to not more than 0.015% and the N content, to not more than 0.015%, and the total content of (C + N) is preferably limited to not more than 0.02%.
• the P content must be reduced to not more than 0.03%, and the P content is preferably as little as possible.
• the S content is preferably less, and must be limited to not more than, 0.01%.
• Mo and W When added to a steel containing 7.5 to 14.0% of Cr, Mo and W are effective for improving the corrosion resistance in a wet carbon dioxide gas environment. In any way, the effect of the addition gets into saturation when they are added in the amount exceeding 3.0%. Further, because large quantities of other alloy elements such as Cu, Ni, Co, etc., must be added so as to form the metallic structure mainly consisting of martensite, heat-treatment of the hot coil becomes difficult. Therefore, the upper limit of each of Mo and W is set to 3.0%.
• this MC value When this MC value is less than 0, it is difficult to form the metallic structure consisting substantially of martensite whichever hot-rolling condition and heat-treatment condition may be selected, and the strength and toughness as the indispensable characteristics for the oil well pipe or the line pipe drop.
• the MC value When the MC value is less than 0, further, it becomes difficult to stably form the austenite structure in the hot rolling temperature zone, the possibility of the occurrence of large rolling scratches becomes high, and the production yield drops. Therefore, the MC value must be at least 0.
• a steel, the metallic structure of which substantially consists of martensite can be obtained by the combination of the later-appearing rolling condition, coiling condition and cooling condition.
• Nb, V and Ti When added to a steel containing 7.5 to 14.0% of Cr, Nb, V and Ti provide a great effect of reducing the hardness of the welding heat affected zone, and also improve the corrosion resistance. However, when they are added in excessive amounts, the effect of addition gets into saturation and the toughness of the base metal drops. Therefore, the sum of at least one of Nb, V and Ti must not exceed 1.0%. Particularly when an excellent toughness of the base metal is required, the sum of at least one of Nb, V and Ti does not preferably exceed 0.5%. On the other hand, in order to sufficiently lower the hardness of the welding heat affected zone, the sum of at least one of Nb, V and Ti is preferably at least 0.1%.
• Rare earth elements and Ca are elements which are effective for improving hot workability and impact toughness. However, when the rare earth elements in an amount more than 0.05% and Ca in an amount more than 0.03% are added, coarse non-metallic inclusions of these elements are formed, respectively, and hot workability and corrosion resistance are deteriorated. Therefore, the upper limit is 0.05% for the rare earth elements and 0.03% for Ca.
• the term "rare earth elements" used in this specification represents the elements having the atomic numbers of 57 to 71, 89 to 103 and Y.
• the steel used for the method of the present invention may contain Zr, B, etc., as mixed impurities from scraps or those added for adjusting toughness, workability, but in such a case, too, the MC value described above must be at least 0.
• the oxygen content is not particularly limited in the present invention, the oxygen content is preferably as small as possible because oxygen is an impurity which forms oxide type non-metallic inclusions.
• Hot workability in hot rolling must be secured by uniformly heating the slab to its center portion. However, if heating is made to a temperature more than 1,300°C, the material loss due to the formation of oxide scales becomes so remarkable that the production yield drops. When the heating temperature is less than 1,050°C, on the other hand, the deformation resistance in hot rolling becomes excessively great. Therefore, the slab heating temperature is limited to 1,050 to 1,300°C.
• Ordinary hot coil rolling can be employed for hot rolling.
• the sheet thickness is limited to at least 3.0 mm to not more than 25.4 mm from practical utility of the sheet for the oil well pipe or the line pipe. From the aspect of productivity in subsequent seam welding, the shape of the sheet is limited to the hot coil.
• the metallic structure substantially comprises the austenite monophase
• the hot rolling finish temperature and to the coiling temperature there are no other limitations to the hot rolling finish temperature and to the coiling temperature. If the temperature is too low, however, the hot rolling deformation resistance becomes great even though the structure is the austenite monophase structure. Therefore, a suitable temperature must be set within the range of the capacity of the hot rolling mill and that of the coiling machine.
• cooling When the hot coil after coiling is cooled, cooling must be carried out at a cooling rate of at least 0.01 °C/sec to a temperature of 500°C or lower. This is for preventing the formation of ferrite from austenite and converting the steel to one whose metallic structure substantially comprises martensite after cooling. If the cooling rate is less than 0.01 °C/sec, the possibility that ferrite is formed during cooling becomes high. In the steel to which the present invention is directed, on the other hand, austenite cooled to less than 500°C no longer undergoes transformation to ferrite, and because the cooling rate at a temperature less than 500°C has small influences on the martensite transformation, any cooling rate may be used at a temperature less than 500°C.
• the heating temperature of less than 550°C or the holding time of less than 15 minutes is not preferable because the toughness of the base metal is not sufficient.
• the heating temperature exceeds the A c1 transformation point, fresh martensite is formed in subsequent cooling process and the toughness as well as the stress corrosion cracking resistance of the base metal drop.
• the holding time of at least 15 minutes is secured, a longer holding time causes no problem.
• box annealing is employed, the holding time is from about 2 to about 10 hours.
• the reheating atmosphere may be the air atmosphere, but is more preferably a non-oxidizing atmosphere or a reducing atmosphere in order to reduce the oxide scales on the steel surface and to improve the production yield of the steel pipe without lowering the corrosion resistance.
• a mixed gas consisting of 5 to 15% of hydrogen and the balance of a nitrogen or argon gas.
• An ordinary production process of an electric resistance seam welded steel pipe can be employed for forming and seam welding in the present invention, and a seam welded steel pipe is produced by cutting a steel coil into a predetermined width in accordance with a required outer diameter as an oil well pipe or a line pipe and welding both edges of the steel coil by electric resistance welding while continuously shaping the steel coil so cut into a cylindrical shape.
• the steps of producing the steel pipe by seam welding, reheating the seam welded portion and the portions within 2 mm from both sides of the seam to a temperature of not less than 550°C and not more than the A c1 transformation point and then cooling the pipe may be added, whenever necessary.
• the object of this additional production step is to lower the hardness of the hardened structure formed locally at the time of seam welding and to improve the toughness of the seam welded portion.
• reheating is carried out, only the portions in the proximity of the seam welded portion may be reheated immediately after seam welding by using a post annealer, for example, or the full-body of the steel pipe may be heated.
• the present invention to add the steps of reheating the seam welded portion and the portions within at least 2 mm from both sides of the seam welded portion to not less than the A c3 transformation point +50°C, rapidly cooling them to a temperature below an Ms point, further heating again at least the seam welded portion and the portions within 2 mm from both sides of the seam to a temperature from 550°C to the A c1 transformation point and then cooling them.
• the object of the additional steps is to reduce non-uniformity occurring at the time of seam welding and to further improve the toughness of the seam welded portion.
• the seam welded portion and the portions within at least 2 mm from both sides of the seam are heated to not less than the A c3 transformation point +50°C, it is preferred to reheat only the portions in the proximity of the seam welded portion immediately after seam welding by using the post-annealer.
• the steel pipe may be naturally heated as a whole, but in this case, the steel pipe is hardened as a whole, so that the material property secured at the time of the hot coil is lost. After reheating is made to the A c3 transformation point +50°C or more, the pipe must be rapidly cooled to a temperature lower than the Ms point.
• the metallic structure of the hot coil of the steel having the selected components is converted to the structure substantially consisting of tempered martensite. If the structure of the hot coil remains un-tempered martensite, the strength is excessively high and hence, workability and toughness are extremely inferior. In contrast, workability of the steel can be improved by tempering the mertensite under the state of the hot coil so as to provide a suitable strength to the hot coil, and forming in the production of the seam welded steel pipe can be attained with a remarkable increase in productivity.
• the metallic structure is converted to the tempered martensite, a high strength such as a yield strength of at least 551 MPa, for example, can be easily obtained, and a high strength and an excellent impact toughness can be obtained, too.
• Each testpiece having a thickness of 3 mm, a width of 15 mm and a length of 50 mm was used for the wet carbon dioxide environment, and was immersed in a 5% aqueous NaCl solution inside an autoclave at a testing temperature of 120°C at a carbon dioxide pressure of 40 atms for 30 days.
• a corrosion rate was calculated from the weight change between the weight before the test and the weight after the test. The unit of this corrosion rate was expressed by mm/y. It is generally believed that if a corrosion rate of a certain material in a certain environment is less than 0.1 mm/y, the material is sufficiently anti-corrosive and can be used.
• test results are also tabulated in Table 2.
• symbol ⁇ shows that a fracture appearance transition temperature is not more than -30°C
• symbol X shows that the fracture appearance transition temperature is from -30°C to 0°C
• symbol XX shows that the fracture appearance transition temperature exceeds 0°C.
• symbol ⁇ shows the maximum hardness is less than 300
• X shows that it is from 300 to less than 450
• symbol XX shows that it is at least 450.
• the present invention can produce, at a low cost and with high productivity, steel pipes excellent in both corrosion resistance and weldability.

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• Chemical & Material Sciences (AREA)
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• Crystallography & Structural Chemistry (AREA)
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• 1994
  • 1994-06-16 JP JP06156494A patent/JP3116156B2/ja not_active Expired - Fee Related
• 1995
  • 1995-06-16 CA CA002192833A patent/CA2192833C/fr not_active Expired - Fee Related
  • 1995-06-16 CN CN95194138A patent/CN1152947A/zh active Pending
  • 1995-06-16 WO PCT/JP1995/001207 patent/WO1995034690A1/fr active IP Right Grant
  • 1995-06-16 EP EP95921979A patent/EP0774520B1/fr not_active Expired - Lifetime
  • 1995-06-16 KR KR1019960707203A patent/KR100206503B1/ko not_active IP Right Cessation
  • 1995-06-16 DE DE69529162T patent/DE69529162T2/de not_active Expired - Lifetime
• 1996
  • 1996-12-13 US US08/750,758 patent/US5820703A/en not_active Expired - Lifetime
  • 1996-12-13 NO NO965386A patent/NO965386L/no not_active Application Discontinuation

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